A. Field of the Invention
The present invention relates to an ultrasound probe to be used with an ultrasound diagnosis apparatus, and more particularly to an ultrasound probe having a forced cooling system for circulating a liquid through a flexible cable connecting the ultrasound probe.
B. Background of the Invention
An ultrasound probe is detachably coupled to an ultrasound diagnosis apparatus for imaging an internal status of an object through ultrasounds. The ultrasound probe includes a handle unit including a tip portion for, making contact on a patient body surface, a cable connected to the handle unit, and a connector unit for connecting the cable to an ultrasound diagnosis apparatus. Usually, to freely operate the handle unit by an operator, a flexible cable having around two (2) meters in length for connecting the handle unit to the connector unit.
In an array type ultrasound probe, particularly in a two-dimensional (2D) array ultrasound probe, ultrasound transmissions and receptions are performed through several thousands of transducers. Recently, in a 2D array ultrasound probe, a part of the ultrasound transmission and reception functions by the ultrasound diagnosis apparatus is executed by an electronic circuit provided in the ultrasound probe. Thus, an electronic circuit in a handle unit of the 2D array ultrasound probe performs ultrasound transmission functions and a part of ultrasound reception functions.
To transmit and receive various signals such as ultrasound data signals, control signals and power supplying signals, between the electronic circuit in the handle unit and the ultrasound diagnosis apparatus main body, a plurality of electric signal lines passes through in the flexible cable. The electric signal lines include power supplying twisted pair wires, control signal lines and very thin coaxial lines for transmitting and receiving ultrasound signals. Typically, more than two hundred of the electric signal lines are included in the flexible cable. Usually, each of the signal lines has an external diameter φ of about 0.3 millimeter. The electric signal lines are twisted together and adjusted so that a cross-sectional plane of a whole cable is configured to a circular shape. The whole cable is covered by a shield member and an outer envelope member.
The electronic circuit in the probe is mounted on a substrate provided in a handle unit as an integrated circuit (IC). During the time of the ultrasound transmissions and receptions, the IC in the handle unit generates heat of several watts (1˜2 W) by consuming electric power. While the heat generated in the handle unit is radiated through the probe housing, a part of the heat is transmitted to a tip portion of the probe. A plurality of ultrasound transducers is mounted in the tip portion of the probe. Consequently, in the ultrasound transmission time, the plurality of transducers also generates the heat. As a result, the heat component transferred from the IC and the other heat component generated in each of the transducers are summed up and increase the temperature of the tip portion of the probe that contacts a patient.
The temperature increase of the tip portion of the probe causes a low temperature burn on the patient. To prevent this, an upper limit of the temperature increase of the tip portion of an ultrasound probe has been determined by the international standard (IEC 60601). While the heat generated from the transducers is proportional to the acoustic energy emitted into a living body, the heat generated from the IC contains a component proportional to the transmission energy and a constant component due to the bias current for the transmission and reception circuit.
Usually, to keep the temperature increase of the tip portion contacting a patient under a limited temperature, the total power consumption of the transducers is controlled by limiting the transmission voltage. In the 2D-array probe having an IC in the handle unit, since the IC generates the heat, it needs to keep a transmission voltage that is lower than that in the normal probe in which an electronic circuit is not provided in order to keep the temperature of the patient contacting portion of the probe under the limited temperature. At the lower transmission voltage, a sound pressure of the transmission ultrasound emitted into the living body is also reduced, and the receiving voltages of echo signals are reflected from the living body become small. As a result, the signal to noise (S/N) ratio is reduced. Further, since a generation ratio of harmonic contents proportional to a square of the transmitting sound pressure also becomes a small, it becomes difficult to improve an image quality by receiving the harmonic contents.
To avoid these problems, an ultrasound probe having a forced cooling system has been proposed for cooling the IC provided in a handle unit by circulating a cooling liquid around the IC (for instance, see Japanese Patent Application Publication 2003-38485). The forced cooling system includes a pump provided in an ultrasound diagnosis main body or a connector unit of an ultrasound probe and two hollow tubes provided in the cable for circulating a cooling liquid in the handle unit through the cable. Thus, as shown in FIG. 5, one of the two hollow tubes is used as a cooling outward path for flowing the cooling liquid from the connector unit to the handle unit and the other is used as a cooling return path for returning the cooling liquid from the handle unit to the connector unit.
Once the cooling liquid has returned to the connector unit after absorbing the heat in the handle unit, it needs to radiate the absorbed heat before re-circulating through the cooling outward path in order to keep the cooling efficiency. Usually, the heat radiation from the connector unit is performed by a heat sink and/or heat radiating fins in the connector unit. Further, an air cooling fan and a heat outfall are provided in the connector unit, if necessary. It is also possible to perform the heat radiation by a heat connecting member on the connector unit connected to the ultrasound diagnosis apparatus so as to directly transfer the heat to the ultrasound diagnosis apparatus having a big heat capacity.
Usually, a rubber tube or a plastic tube made of an urethane resin or a fluorine resin is used as the circulating cooling liquid flow channel. An internal diameter φ of the flow channel is around 1.5 to 2.0 millimeters in accordance with a required liquid flow amount and a pumping power of the pump provided in the connector unit. To perform various diagnoses by operating the handle unit at various angles and positions, the cable should be flexible. Thus, the cable is bent and deformed in accordance with the operation of the handle unit. If the tube strength is insufficient, the tube is bent by the deformation of the flexible cable. As a result, the temperature of the patient contacting portion increases due to the insufficient amount of the cooling liquid supply. This is a serious problem.
To avoid this problem, it needs to keep the wall thickness of the cooling tube more than about one (1) millimeter. As a result, usually, an external diameter φ of the cooling tube becomes about 3.5 to 4.0 millimeters. Thus, the cable is needed to include two tubes for a cooling liquid outward path and a liquid return path in it, and each tube has an external diameter φ of about 4.0 millimeters.
However, when the flexible cable is manufactured by including a plurality of electric signal lines together with the two cooling tube in it, the difference in hardness and diameter between the electric signal line and the cooling tube generates another problem. Typically, the cooling tube having a relatively thick external diameter φ of four (4) millimeters is harder than the electric signal lines, each having a smaller diameter. As a result, it becomes difficult to easily twist the thin electric signal lines together with the cooling tube. Further, the hardness of the cooling tubes affects the flexibility of the ultrasound probe. Thus, a free operability of the probe is badly influenced.
Further, when the cable is sharply bent during the operation, it happens to pinch or bend the tube filled with the circulating liquid in an inner flow cavity. In such a case, since the amount of the cooling liquid is extremely reduced, the temperature of the tip portion of the probe is largely increased. As explained before, the temperature increase of the tip portion causes a low temperature burn of the patient. As the wall thickness of the cooling tube is increased to avoid the buckling, the flexibility of the cable is decreased. Further, since the external diameter of the cable is also increased, the operability of the probe is extremely reduced.